Hello
My customer is using PCM1795 for thier CD-P several years for their products, and they intends
to use for their 2015model also ,
However, recently, they found the THD had 7dB worsen(103 to 96.5dB(0.0015%) compared with
old Lot (2year ago) in their pre-production..
When they had replaced the old stock in their PCB on Pre-Production, THD improved.
They are concerned about any change in the PCM1795 production..
Cusomter's setting; is following and did not change basic design from past 3years.:
Input 44.1KHz(CD)
x8 oversampling : using customer's own external digital filter(DSP)
WDCLK= 705.6KHz
BCLK= 22.5792MHz(512fs)
Sys CLK(SCK)=512fs
DFMS=0 (MONO)
Q1: First of all, I think customer's usage in the following is no problem , isn't it?
SCK = fs * OSR= fs * OSRint * OSRext.
For example, fs = 44.1kHz, OSRint = 32x, OSRext = 16x,
therefore SCK = 22.5792MHz.
I think it seems No problem from the Data sheet Table3(Page21), 512fs description .
How do you think about this?
I find following in the forum also..
https://e2e.ti.com/support/data_converters/audio_converters/f/64/p/312810/1089142#1089142
Q2 Please advise about the relationship between THD~ OSR in case external digital filter mode?
For example, THD/SNR is in inverse propotion to OSR? (For example, 3dB worsen x2 times OSR ? customer think so )
Or, there are no relationship?
Q3 What valuse of THD should be for customer's setting( WCLK=768KHz)?
0.0015% is make sence ?
Q4 Please advise about the relationship between SNR~ OSR in case external digital filter mode